Yard jib crane

ABSTRACT

A yard jib or tower crane having a vertical mast, a hoist boom supporting a load hoist truck being pivotally supported for rotation about the mast end including a first beam and a second beam attached to said first beam and being radially movable relative thereto; a counterweight boom supporting a balancing weight compensating for the moments about the mast generated by said truck-supporting boom; and a control including a replaceable profile cam for predetermined radial displacement of said second beam relative to the first beam and of the balancing weight along the counterweight boom during angular displacement in a horizontal plane of said boom about the mast.

United States Patent 11 1 Durand 1451 Aug. 7, 1973 YARDJIB CRANE [75] Inventor:

[22] Filed: Jan. 22, 1971 [21] Appl. No.: 108,753

Pierre Durand, Lyon, France [30] Foreign Application Priority Data FOREIGN PATENTS OR APPLICATIONS 3/1960 France 212/48 Primary Examiner-Richard E. Aegerter Assistant Examiner-Merle F. Maffei Attorney-Waters, Roditi, Schwartz & Nissen [57] ABSTRACT A yard jib or tower crane having a vertical mast, a hoist boom supporting a load hoist truck being pivotally supported for rotation about the mast end including a first beam and a second beam attached to said first beam and being radially movable relative thereto; a counterweight boom supporting a balancing weight compensating for the moments about the mast generated by said truck-supporting boom; and a control including a replaceable profile cam for predetermined radial displacement of said second beam relative to the first beam and of the balancing weight along the counterweight boom during angular displacement in a horizontal plane of said boom about the mast.

10 Claims, 12 Drawing Figures PAIENIEB M18 7 1975 SHEEI 1 0F 4 PAIENIEB SHEEI 2 BF 4 PATENTEB SHEET 3 UF 4 u L 3 8 mm i 9 m s T Um I w w E YARD JIB CRANE The present invention relates to yard jib or tower cranes and the like.

Any limitation of the angular excursion of the crane boom about a crane mast supporting the boom reduces the crane versatility, the area covered by its hoisting means, and, sometimes, the efficiency of the crane itself. In order to avoid the necessity of limiting this rotation, contractors presently use giant cranes leaving under their booms spaces which are considerably greater than to the height of the buildings or other structures and throughfares above which the boom is being operated. However, in that case, considering the excessive length of hoisting rope required, the efficiency of the work performed near ground level is considerably reduced, and contractors are confronted with the problem of causing a boom hoist to move over inhabited houses and busy thoroughfares.

SUMMARY OF THE INVENTION The novel yard jib or tower crane according to this invention avoids these various inconveniences encountered in the prior art. 7

According to this invention, the hoist boom of the crane consists of a main beam which is connected to the head or top of the tower or mast, and of a secondary beam attached to and adapted to be moved radially in either direction in relation to the first or main beam through suitable control or drive means. The latter are adapted to also control elements, such as tackles, blocks, or the like, which cause a counterweight to move in a radial direction opposed to the direction of movement of the secondary beam, so that the crane moment, under no-load conditions, will not be appreciably changed by the displacement of the secondary beam along the first beam, the foregoing procedure being effected without producing any change in the radial position of the hoist truck in relation to the tower mast head.

Thus, whatever the angular position of the crane may be as measured in the horizontal plane about the mast, it is extremely easy to impart the desired operating length to the hoist boom. Moreover, this arrangement permits imparting to both the first and second hoist beams the overall relative length dimensions consistent with the loads supported thereby. Thus, the stress generated in the secondary beam will be considerably lower than that generated in the main beam, in view of which the secondary beam may be constructed of smaller dimensions and weight than the main beam. Another advantage of this arrangement lies in that the crane can be operated under the greatest range of conditions while taking the utmost advantage of its specific characteristics.

According to a preferred form of embodiment of the present invention, the element controlling the radial movements of the secondary beam are used, on the one hand, in combination with a member which is supported against relative rotation with respect to the jib, so as to reproduce the desired path or trajectory of the free end of the secondary beam during a 360 degree rotation, measured in the horizontal plane, of the complete jib assembly about the mast, and, on the other hand, includes a replaceable profile cam which is adapted to detect, on the aforementioned element and for each angular position of the jib, the radial position relative to the first beam to be imparted to the secondary beam, and also to concurrently actuate the control element for automatically setting the secondary beam in its proper operating position.

With this arrangement, as the jib is angularly slewed in relation to the tower mast, its free or outer end automatically moves so as to follow the path imparted thereto by the predetermined working profile or outline provide for on the replaceable profile cam.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate generally schematically by way of example preferred embodiments of the crane. In the drawings FIG. 1 is a side elevational view of the upper portion of the crane showing the arrangement for hoisting the load;

FIGS. 2 and 3 are fragmentary views, on an enlarged scale, showing more particularly in, respectively a side elevational view and in a section taken along line 3-3 of FIG. 2 the arrangements for moving the secondary beam in relation to the main hoist beam, and the hoist truck in relation to both beams FIG. 4 is a side elevational view showing various phases of the transfer of the hoist truck from the secondary beam to the main hoist beam and reversely FIGS. 5 and 6 are fragmentary views similar to FIGS. 2 and 3, i.e. taken, respectively, in side elevation and section along line 6-6 of FIG. 5, showing a modified arrangement for guiding the hoist truck on the main and secondary beams FIGS. 7 and 8 are sectional views taken along, respectively, lines 7-7 and 8-8 of FIG. 6, showing an arrangement for laterally centering the hoist truck of FIGS. 5 and 6 during its transfer to the secondary beam and to the main beam, respectively FIGS. 9 and 10 are schematic views showing two arrangements of the device for moving the secondary beam in relation to the main beam, and

FIGS. 11 and 12 are schematic views, respectively showing two arrangements of the rope means for driving the hoist truck.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, the reference numeral 2 designates the tower of a jib crane, including a mast head 3 which is rotatably mounted on the upper por tion of this tower. On either side of the mast head 3, a counterweight boom 4 carrying a counterweight 5 and a hoist or truck boom which is generally designated as 6, are provided in aligned radial arrangement relative to the vertical axis of the tower 2..

' According to the present invention, the hoist or truck boom 6 comprises two elementl, i.e. i a main beam 7 having its root or base portion connected to the crane head 3,

and a secondary beam 8 extending parallel to and below the main beam and adapted to be moved in coextensive radially movable relation therewith.

It becomes obvious that independently of the specific features to be described hereinbelow this construction permits imparting to each beam 7, 8, relative axial dirnensions and displacements consistent with the objects or loads to be supported thereby.

As shown in greater detail in FIGS. 2 and 3, the secondary beam 8 is carriedby the main beam 7 through the intermediary of identical sets of rollers, such as, respectively, upper and lower sets of rollers 9a and 9b. These sets of rollers are in turn carried by the lower frame structure of the main beam 7, adjacent the outer or free end thereof. Tracks are provided extending the length of the upper frame member 12 of the secondary beam 8 so as to permit the sliding movement thereof between the sets of rollers 90 and 9b.

In the form of embodiment illustrated in FIGS. 2 and 3, the lower frame member 10 of main beam 7 also constitutes a track for a set of rollers 13a and 13b which are located at the upper portion of a hoist truck 14. This hoist truck is equipped with another set of rollers a and 15b which are adapted to engage the track provided on the upper frame member 12 of secondary beam 8.

From the foregoing it becomes apparent that the truck 14 can travel along the main beam 7 as well as along the secondary beam 8 so as to be able to assume various radial positions between the base end of the former and the free or outer end of the latter, irrespective of the position of the secondary beam.

In order to permit a gradual, smooth passage or transfer of the truck 14 from the main beam 7 to the secondary beam 8 and conversely, the pairs of rollers 13b and 15a are shifted vertically upwards in relation to their homologues 13a and 15b. Moreover, the distance e1 between centers of rollers 13a and 15a is determined to be slightly lesser than the distance e2 between the pairs of rollers 13b and 15b. In addition, inclined surfaces 10a and 12a are provided, respectively, at the boom head end of the lower frame member 10 of main beam 7, and at the root or base end of the upper frame member 12 of secondary beam 8.

When the truck 14 is supported concurrently on the main beam 7 and on the secondary beam 8, as illustrated in FIG. 2 and at A in FIG. 4, ther is provided a clearance J1 between the rollers 15a and the registering faces of the tracks of frame member 12, and a clearance J2 between the rollers 13b and the registering faces of the tracks of frame member 10. On the other hand, it is noted that these clearances J1 and J2, as well as the distances between ceters el and e2, are calculated as a function of the gradient of the inclined surfaces 10a and 12a.

In the position denoted by A in FIG. 4, the truck 14 is carried both by the main beam 7 through rollers 13a and by the secondary beam 8 through rollers 156.

When the truck is moved in the direction of the arrow 16 (FIG. 4) it simultaneously rolls on the two tracks of frame members 10 and 12 through the intermediary of the rollers 13a and 15b, until the rollers 13a begin to move down the incline 10a. As the rollers 13a roll along this incline a the clearance J1 between rollers 15a and the track of frame member 12 decreases and becomes zero when the rollers 15a engage the lastmentioned track. At that time, as shown at B in FIG. 4, the rollers 13a no longer bear on frame member 10, and the truck 14 is then carried only by the secondary beam 8 through rollers 15a and 15b, upon which it then continues its movement along the secondary beam.

When the truck 14, in the position shown in FIG. 2, moves in the direction of the arrow 17, it rolls first by means of rollers 13a and 15b along frame members 10 and 12, and continues in this manner until the rollers 15b begin to move down the incline 12a. As the rollers 15b roll down the incline the clearance J2 existing between the rollers 13b and the raceway of frame member 10 decreases and eventually disappears as the rollers 13b engage the aforesaid track. At this time, as shown at C in FIG. 4, the truck 14 is carried only by rollers 13a and 13b and afterwards continues its movement on these rollers along the main beam 7.

When the truck 14 travels from position B or C to position A (FIG. 4), the movements take place exactly in a reverse order with respect to the sequence described hereinabove, rollers 13a and 15b controlling by their engagement with inclines 10a and 12a the correction of the truck position so that only these rollers 13a and 15b engage their cooperative tracks.

Thus, with this arrangement it is possible to transfer the truck from one beam to another with the maximum progression and degree of smoothness.

In a modified arrangement as illustrated in FIGS. 5 to 8 of the drawings, the rollers 15a and 15b of truck 14 travel on a lower I-section frame member 18 of secondary beam 8, and with the upper frame member 12 of this beam travelling between the sets of rollers 9a and 9b. Rollers 13a, 13b and 15a, 15b are, of course, disposed similar to those of the preceding arrangement so as to permit the desired progression of the passage of the truck from one beam to another.

Moreover, the truck 14 is provided with lateral flanges 19 at its end closest to the outer end of the boom, in order to properly position the truck laterally in relation to the frame member 18 and to facilitate the passage of the truck from the main beam to the secondary beam. As illustrated in FIG. 7, these flanges 19 are spaced by a distance L slightly greater than the width 1 of frame member 18 and comprise, at their free ends, diverging or outflaring outwardly flaring portions which act like guide members.

Similarly, the truck 14 is provided with rollers 11 movable about vertical axis and adapted to roll along the inner surface of frame members 10. When the truck is being transferred from the secondary beam to the main beam, it is laterally positioned by these rollers 11 which, as shown in chain-dot lines in FIG. 8, engage end ramps 10b formed at the free or head end of frame members 10 and on the surfaces registering therewith.

The movements of the secondary beam below the main beam may be controlled through any suitable and known electrical, hydraulic or other actuating means.

In a specific form of embodiment, illustrated by way of example in FIG. 9, the secondary beam is driven by a mechanism comprising a winch 22 which is mounted, for instance, on the main beam 7 which is and driven in turn by an electric motor 23.

The winch 22 is of the double-winding drum type so as to control the movements of two runs 24 and 25 of a suitable rope. Run 24 is anchored at 26 to the secondary beam 8 and passes over a return pulley 27 mounted on the head or free end of the main beam 7. The other run 25 passes over various pulleys constituting the set of pulleys 28a and 28b of a tackle 28 prior to being anchored with its free end, at 29, to the counterweight boom 4. The set of pulleys 28a of tackle 28 is restrained against translation along the counterweight boom 4 and the other set 28b is connected to the counterweight 5 proper so as to be adapted to travel along the boom 4 in either direction, in correspondence with to the direction in which the secondary beam is travelling. The counterweight 5 is operatively connected to a second set of pulleys 30a of a tackle 30 of which the first group 30b is restrained against translation along the counterweight boom 4. A rope 32 passing over the pulleys of this tackle is attached at one end 33 to the counterweight boom 4 and at its opposite end 34 to the secondary beam 8.

Preferably, the number of runs formed in each tackle 28, 30 is proportional to the ratio of the balance counterweight 5 to the weight of the secondary beam 8.

With this novel arrangement, for each extent of travel of the secondary beam 8 in one direction there is provided a proportional extent of travel in the opposite direction of counterweight 5, whereby the no-load moment of the crane can be permanently maintained at a nearly constant value, irrespective of the radial position of the secondary beam.

The mechanism driving the secondary beam may be controlled directly by the crane driver as a function of the configuration of the area above which the truck boom is to be operated, however, in a preferred form of the embodiment illustrated schematically in FIG. 9, this mechanism is automatically controlled. To this end, the energization of the electric motor 23 is controlled by an apparatus 35 receiving information from a reading unit 36 travelling in conjunction with the crane jib relative to a stationary replaceable profile cam 37. The edge of the profile cam 37 is dimensioned to provide a profile configuration having the same pattern as the ideal path, of movement, measured in a horizontal plane, of the free end of the secondary beam during a 360 displacement of this beam about the vertical axis of the mast head 3.

With this arrangement, each angular movement of the jib is attended, if necessary, by a concurrent radial displacement of the secondary beam in relation to the main beam, this travel depending on the orientation of the truck boom and permitting the movement of the secondary beam either radially outwardly or inwardly in order to take the best possible advantage of the geographical area over which the truck boom is being operated, while preventing the boom from moving over thoroughfares or striking existing buildings which rise at least as high as the crane tower.

Thus, the yard jib or tower crane can be operated at a maximum degree of efficiency on any building sites, even on a site deeply situated among occupied houses or juxtaposed above busy thoroughfares,,requiring only very short preparation in time necessary for preparing a profile cam having a profile homothetic to the ideal trajectory to be followed by the free end of the secondary beam under the above-defined geographical condipulleys 28b and 30a of tackles 28 and 30 respectively,

of which the number of runs is equal to n, i.e. to the ratio of the weight of counterweight 5b to the weight of secondary beam 8.

In this crane, the truck driving means must be so arranged that the relative movement of the secondary beam to the main beam cannot interfere with the truck position. To this end, the truck driving means, as shown in FIG. 1 1, comprises a winch 38 mounted for instance on the counterweight boom 4 and has a double winding drum from which extend two runs 39 and 40 of a suitable rope.

The first run 39 passes over a pulley 42 which is mounted on the head or outer end of main beam 7, then returns below this beam so as to pass over another pulley 43 which is disposed at the root of the secondary beam, and adapted to return the run 39 to a third pulley 44 which is mounted at the outer end of the secondary beam 8. From pulley 44 the run is returned to the root or base of beam 8, and finally its free end is attached to the truck 14. The other run 40 passes over a return pulley 45 which is mounted, for instance, on the counterweight boom 4 and has its free end similarly anchored to the truck 14.

It is apparent that the return pulleys 42 and 43 constitute a kind of block which, by automatically compensating for the movement of the secondary beam 8, allow of maintaining the truck 14 in the position initially imparted thereto before moving the secondary beam 8. g

As in a conventional tower crane the load hoisted by the crane of this invention must be maintained in its preset position during the movement either of truck 14 or of secondary beam 8. To this end, the device controlling the load movement comprises, as shown schematically in FIG. 1, a hoist 46 from which extends a rope 47 passing firstly over the pulleys 48, 49 and 50 of a set disposed near or at the root of the main beam, and then successively over the truck pulley 52, the pulleys 53 of the tackle block supporting the load or crane book 54, and finally over another pulley 55 of truck 14. Hence, the rope passes over pulleys 56 and 57 which are mounted on, respectively, the head and root or base of the secondary beam 8, and its free end is eventually anchored at 58 to the head or free end of the main beam 7.

According to a modified form of embodiment the truck 14 travels only below the secondary beam 8. In this case the radial displacement of the load takes place from the minimum load radius to the maximum load radius of main beam 7, by moving the secondary beam 8 having the truck 14 locked against motion, to the inner end of this beam 7 from the maximum load radius of main beam 7 to the maximum load radius of the truck supporting boom or I jib boom 6, by moving the truck 14 along the secondary beam 8.

The mechanisms controlling the movements of the secondary beam and the movement of the load are exactly the same as those described hereinabove with reference to the preceding form of embodiment; only the truck driving mechanism differs since the truck must be capable of preserving the position imparted thereto on the secondary beam even when the latter is moved along the boom. This as illustrated in FIG. 12 comprises a winch drum 60 which is mounted on the secondary beam 8, and two rope runs extending from this drum, one run having its free end attached to the truck at 62 and passing over a return pulley 63 mounted to the root of the secondary beam 8,. and another run attached by its free end at 64 to the truck 14 and passing over another return pulley 65 mounted on the head of the secondary beam 8.

In another arrangement of this construction the secondary beam 8 carries on its lower portion and along its entire length a track in the form of a single rail, consisting of, for example, an I section member 18 (FIG. which is engageable by the rollers of truck 14.

It will readily be obvious to those skilled in the art that the foregoing should not be construed as limiting the scope of the invention to the specific forms of embodiment of the tower crane which are given herein, since many modifications and variations may be effected thereto without departing from the basic principles of the invention, irrespective of the specific type of tackle blocks, rope arrangements and control means utilized therefor.

What is claimed as new is:

1. Improvements in yard jib or tower cranes with a radially travelling hoist truck, of the type including a vertical tower having a mast head and a jib assembly rotatably mounted on the mast head of the tower, said jib assembly comprising a hoist truck boom, a load hoist truck being movable along said hoist truck boom, said truck having a tackle, and a counterweight boom carrying a balance weight adapted to be moved radially relative to said counterweight boom for compensating the moment created at the mast head level by the hoist truck boom, said hoist truck boom further including a main beam connected to the tower mast head and a secondary beam adapted to be moved radially in either direction relative to said main beam, control and driving means for imparting said movement to the secondary beam; tackle means adapted to be actuated by said control and driving means for causing the movement of said counterweight in a direction opposed to the direction of movement of said secondary beam, said control and drive means controlling the movements of the secondary beam being operative in one position thereof with a member held against rotation in relation to the truck boom and adapted to reproduce the desired path of the free end of the secondary beam during 360 rotation in a horizontal plane of the jib assembly about the mast head, and being operative in another position thereof with replaceable profile cam means, said cam means detecting on said element and for each angular position of said jib assembly relative to said mast head the optimum radial position to be imparted to said secondary beam, and further control means adapted to be actuated by said replaceable profile cam means, said last mentioned control means being operatively connected with said secondary beam for automatically setting the latter in said optimum radial position, whereby the no-load moment of the crane remains substantially unchanged during the movement of said secondary beam which takes place without any appreciable change in the radial position of the truck in relation to the tower mast head.

2. Tower crane as set forth in claim 1, comprising rollers disposed on either one of said main beam or secondary beam and engaging a suitable track means carried by the other beam for guiding said secondary beam during its translation in relation to said main beam.

3. Tower crane as set forth in claim 1, the means controlling the movement of said secondary beam in relation to the main beam comprising a double-winding winch drum, means for actuating said winch drum, at least one rope having its two runs adapted to be partly wound on said drum and having free ends attached, respectively, to said secondary beam and to a fixed point of the counterweight boom, a set of return pulleys over which said rope runs pass being carried by the end of said counterweight which is closest to the tower, a further set of pulleys forming a tackle with said set of re turn pulleys being held against translation on said counterweight boom, a third set of pulleys carried on the other end of said counterweight, a fourth set of pulleys held against translation on said counterweight boom forming another rope tackle with said third set of pulleys and having its rope anchored to said counterweight boom through the end of one of its runs and to said secondary beam through the end of its other run, each said tackle comprising a number of runs equal or substantially equal to the ratio of the weight of the counterweight to the weight of the secondary beam.

4. Tower crane as set forth in claim 3, wherein said counterweight comprises a fixed element having a predetermined weight so as to balance the moment resulting from the product of the length of the main beam by one-half of the maximum load likely to be carried by said main beam and of a movable element of which the weight is a multiple of the weight of said secondary beam, said movable element of the counterweight being connected on the one hand to said secondary beam and on the other hand to said counterweight boom through tackle equal or substantially equal in number to the ratio of the weight of said movable counterweight element 'to the weight of said secondary beam.

5. Tower crane as set forth in claim 3, wherein the hoist rope is unwound from a winch disposed at a fixed location on the boom and, after passing over a plurality of said return pulleys, returned to the free end of the main beam towards the hoist tackle carried by the truck and then directed towards a pulley disposed at a free end of said secondary beam and adapted, by forming a tackle with another pulley disposed at the root of said secondary beam, to return to said last-named pulley and subsequently be directed towards a free end of said main beam to which the free end of the run is anchored, whereby the movement of said secondary beam cannot interfere with the vertical position of the load, irrespective of the truck position along the beam.

6. Tower crane as set forth in claim 5, wherein said load truck comprises two sets of rollers, one set of said rollers engaging a track carried by said main beam, and the other set of rollers engaging a track carried by said secondary beam.

7. Tower crane as set forth in claim 6, wherein a first pair of coaxial rollers disposed on the truck end nearest to the head of the boom and forming part of the set of rollers engaging the track of the main beam'is shifted vertically upwards by a distance which is substantially equal to the upward shift of a second pair of coaxial rollers disposed at the tower end of said truck and forming part of the set of rollers engaging said secondary beam, said distance being determined as a function of the gradient of inclined surfaces provided at the outer end of said main beam and at the inner end of said secondary beam, respectively, so that the transfer of said truck from one beam to the other can take place smoothly, irrespective of the defect of parallelism of the tracks carried by these two beams.

8. Tower crane as set forth in claim 4, wherein said load truck comprises only one set of rollers engaging a single track carried by said secondary beam.

9. Tower crane as set forth in claim 6, wherein said load truck driving means comprise a winch drum of the double winding type and a rope wound on said drum and having two runs, one run being attached with its free end to said truck and passing over a return pulley carried by said counterweight boom, the other run of said rope passing in succession over a pulley disposed at the head of said main beam and over another pulley disposed at the root of said secondary beam to constibeing anchored to said truck.

1! l l III 

1. Improvements in yard jib or tower cranes with a radially travelling hoist truck, of the type including a vertical tower having a mast head and a jib assembly rotatably mounted on the mast head of the tower, said jib assembly comprising a hoist truck boom, a load hoist truck being movable along said hoist truck boom, said truck having a tackle, and a counterweight boom carrying a balance weight adapted to be moved radially relative to said counterweight boom for compensating the moment created at the mast head level by the hoist truck boom, said hoist truck boom further including a main beam connected to the tower mast head and a secondary beam adapted to be moved radially in either direction relative to said main beam, control and driving means for imparting said movement to the secondary beam; tackle means adapted to be actuated by said control and driving means for causing the movement of said counterweight in a direction opposed to the direction of movement of said secondary beam, said control and drive means controlling the movements of the secondary beam being operative in one position thereof with a member held against rotation in relation to the truck boom and adapted to reproduce the desired path of the free end of the secondary beam during 360* rotation in a horizontal plane of the jib assembly about the mast head, and being operative in another position thereof with replaceable profile cam means, said caM means detecting on said element and for each angular position of said jib assembly relative to said mast head the optimum radial position to be imparted to said secondary beam, and further control means adapted to be actuated by said replaceable profile cam means, said last mentioned control means being operatively connected with said secondary beam for automatically setting the latter in said optimum radial position, whereby the no-load moment of the crane remains substantially unchanged during the movement of said secondary beam which takes place without any appreciable change in the radial position of the truck in relation to the tower mast head.
 2. Tower crane as set forth in claim 1, comprising rollers disposed on either one of said main beam or secondary beam and engaging a suitable track means carried by the other beam for guiding said secondary beam during its translation in relation to said main beam.
 3. Tower crane as set forth in claim 1, the means controlling the movement of said secondary beam in relation to the main beam comprising a double-winding winch drum, means for actuating said winch drum, at least one rope having its two runs adapted to be partly wound on said drum and having free ends attached, respectively, to said secondary beam and to a fixed point of the counterweight boom, a set of return pulleys over which said rope runs pass being carried by the end of said counterweight which is closest to the tower, a further set of pulleys forming a tackle with said set of return pulleys being held against translation on said counterweight boom, a third set of pulleys carried on the other end of said counterweight, a fourth set of pulleys held against translation on said counterweight boom forming another rope tackle with said third set of pulleys and having its rope anchored to said counterweight boom through the end of one of its runs and to said secondary beam through the end of its other run, each said tackle comprising a number of runs equal or substantially equal to the ratio of the weight of the counterweight to the weight of the secondary beam.
 4. Tower crane as set forth in claim 3, wherein said counterweight comprises a fixed element having a predetermined weight so as to balance the moment resulting from the product of the length of the main beam by one-half of the maximum load likely to be carried by said main beam and of a movable element of which the weight is a multiple of the weight of said secondary beam, said movable element of the counterweight being connected on the one hand to said secondary beam and on the other hand to said counterweight boom through tackle equal or substantially equal in number to the ratio of the weight of said movable counterweight element to the weight of said secondary beam.
 5. Tower crane as set forth in claim 3, wherein the hoist rope is unwound from a winch disposed at a fixed location on the boom and, after passing over a plurality of said return pulleys, returned to the free end of the main beam towards the hoist tackle carried by the truck and then directed towards a pulley disposed at a free end of said secondary beam and adapted, by forming a tackle with another pulley disposed at the root of said secondary beam, to return to said last-named pulley and subsequently be directed towards a free end of said main beam to which the free end of the run is anchored, whereby the movement of said secondary beam cannot interfere with the vertical position of the load, irrespective of the truck position along the beam.
 6. Tower crane as set forth in claim 5, wherein said load truck comprises two sets of rollers, one set of said rollers engaging a track carried by said main beam, and the other set of rollers engaging a track carried by said secondary beam.
 7. Tower crane as set forth in claim 6, wherein a first pair of coaxial rollers disposed on the truck end nearest to the head of the boom and forming part of the set of rollers engaging the track of the main beam is shifted vertically upwards by a distancE which is substantially equal to the upward shift of a second pair of coaxial rollers disposed at the tower end of said truck and forming part of the set of rollers engaging said secondary beam, said distance being determined as a function of the gradient of inclined surfaces provided at the outer end of said main beam and at the inner end of said secondary beam, respectively, so that the transfer of said truck from one beam to the other can take place smoothly, irrespective of the defect of parallelism of the tracks carried by these two beams.
 8. Tower crane as set forth in claim 4, wherein said load truck comprises only one set of rollers engaging a single track carried by said secondary beam.
 9. Tower crane as set forth in claim 6, wherein said load truck driving means comprise a winch drum of the double winding type and a rope wound on said drum and having two runs, one run being attached with its free end to said truck and passing over a return pulley carried by said counterweight boom, the other run of said rope passing in succession over a pulley disposed at the head of said main beam and over another pulley disposed at the root of said secondary beam to constitute a tackle, and thereafter about another pulley mounted to a free end of said secondary beam before returning towards the truck to which the free end of said other run is attached.
 10. Tower crane as set forth in claim 7, wherein said truck driving means comprise a double-winding winch drum from which two rope runs extend, each run passing over a return pulley disposed for one run at a first end of said secondary beam and for the other run at the other end of the same beam, the free ends of said runs being anchored to said truck. 